Cast aluminum is a single phase material in which other elements that are added to the aluminum phase are in solution or dissolved in the aluminum. When aluminum is allowed to slowly cool from a melt phase, e.g., standing in open air, the added elements in the aluminum precipitate out of the solution through a process known as nucleation. Nucleation in material allowed to slowly cool is a process in which not many nuclei are formed, but the ones that do form grow rapidly in size and consume the added elements. This results in a bulk article wherein the aluminum is a relatively pure metal with small volumes of added elements distributed throughout the aluminum. This state is undesirable when the aluminum is being used to form structural articles because pure aluminum is soft and weak.
If aluminum with added elements is rapidly cooled (quenched) while still hot and in a solutionized state, the added elements do not have the opportunity to form large nuclei, but will form many more smaller nuclei as the metals solidify. If the aluminum and added elements have been allowed to slowly cool to a solid, it is still possible to add enough thermal energy to force the added elements to dissolve in the aluminum (solutionize) without actually melting the aluminum. The advantage this offers is that aluminum can be cast into shape, cooled in air until some time later, and then resolutionized and quenched. The thorough distribution of many nuclei, distributed uniformly, dramatically increases the strength of the alloy over pure aluminum or alloys that cool too slowly and form fewer, but large nuclei.
The aluminum alloy can be developed to further improve its mechanical strengths by growing the size of many uniformly distributed nuclei through a process called "aging." During "aging," the nuclei grow as a diffusion process that progresses more rapidly at elevated temperatures. But, if the temperatures are elevated too extensively, the nuclei will collapse together and form fewer large nuclei similar to that found in aluminum that has been slowly cooled, e.g., without solutionizing, as described above.
The aluminum casting industry uses heat treating as a mechanism to increase the strength of the aluminum castings. The process usually amounts to the addition of sufficient thermal energy to force all of the elements that have been added to the aluminum into solution (solutionizing). Energy is consumed as these added elements are dissolved into the aluminum solid phase. The amount of energy required to achieve the necessary diffusion is significant.
Conventional methods for producing cast aluminum alloy products include initially pouring a suitable molten aluminum alloy into a mold. After the molten alloy has completely solidified, the casting is removed, and is set aside to cool in the open air. Normally, a few days worth of production is collected for a batch solutionizing process. Alternatively, the removed casting can be immediately subjected to a solution heat treatment without cooling first.
A conventional method for solution heat treating a cast part involves placing many cast parts in a large forced air convection oven. In the convection oven, the castings are heated to a desired "solution" temperature (approximately 1,000.degree. F.) and maintained at this temperature for at least 2-8 hours. Following the solution heat treatment, the cast part is immediately quenched in water to rapidly cool the product. Following this cooling, the part is naturally or artificially aged.
One of the drawbacks of conventional solution heat treatment processes, such as that described above is the length of time required to complete the treatment. Typically, large numbers of cast aluminum parts are solution heat treated at once in a batch process. Since it is difficult to maintain even and uniform temperatures in all the parts, in order to ensure that all the parts are properly heat treated, the length of the solution heat treatment process is usually at least two hours and often times more than eight hours. The length of time required for the solution heat treatment contributes significantly to the speed with which cast parts can be manufactured and also contributes significantly to the overall energy costs associated with the solution heat treatment process.
It has been proposed that infrared heat treatment systems may improve the operational efficiency of conventional air driven solution heat treating processes by reducing cycle times. For example, U.S. Pat. No. 5,306,359 describes a method for heat treating an aluminum part by applying infrared radiation directly from a source of infrared energy to the part until the part attains a desired state of heat treatment. According to the '359 patent, during the heat treating, the temperature of the part is monitored and the intensity of the radiation source is proportionally controlled in response to the monitored temperature. The temperature of the part in the '359 patent is described as being monitored by a plurality of optical pyrometers 80, 82 and 89, illustrated as being directed toward an irradiated surface of the part.
The '359 patent describes that the use of optical pyrometers to measure the temperature of the aluminum cast parts is complicated by the reflectivity of aluminum and the uncontrolled radiant energy from the background (i.e., the temperature of the lamps, and refractive surfaces). Reportedly, the reflectivity of the aluminum and the radiant energy of the background cooperate to create a temperature readout from the optical pyrometers that is not representative of the temperature of the surface of the part being observed by the optical pyrometers. In an effort to account for these inaccuracies and provide a more accurate reading of the temperature of the part, the '359 patent describes the taking of measurements from a background optical pyrometer, and making adjustments to the readout from the part optical pyrometer based on the readout from the background optical pyrometer.
U.S. Pat. No. 5,336,344 describes a method and apparatus for producing a cast aluminum part using a high intensity electric infrared heating system to heat the part. The described system is similar to the system described in U.S. Pat. No. 5,306,359 noted above. The '344 patent broadly describes that each infrared heating station includes means for monitoring the actual temperature of the wheel, and that the heating of the wheel at each station is controlled in accordance with this monitored temperature. Like the '359 patent, the '344 patent describes that optical pyrometers 46 can be used to generate a signal representative of the wheel temperature. The '344 patent describes that this signal can be used to control the heating of the parts. In the illustrations, the optical pyrometers are shown as being directed at a surface of the part that is irradiated.
U.S. Pat. No. 5,340,418 by the same inventor of the '344 patent proposes additional control methods to control the amount and application rate of infrared energy applied to the part during the solution heat treating process. These proposed methods rely upon the same optical pyrometers described in the '344 patent for assessing the part temperature. In one embodiment, the optical pyrometers are used to monitor the temperature of the part. This temperature is compared to a predetermined solution heat treatment temperature which is chosen as a function of the particular material used to cast the part. As long as the temperature of the wheel as measured is less than the predetermined solution heat treatment temperature, the heating is continued at the initial predetermined level provided by the infrared energy source.
In each of the processes described in the three patents noted above, the cast aluminum part is indexed through a plurality of individual stations while the part is rotated relative to the path of travel. By indexing the part through the stations, the part resides in each station for a predetermined period of time before it is transported to the next station.
Industry expectations for each of the processes and apparatuses described in the patents noted above was high; however, practical experience has shown that the processes and apparatuses described in the above patents have not found commercial acceptance due to difficulties in producing cast aluminum parts with reliable physical properties, such as strength. Accordingly, there continues to be a need for improvements to processes for solution heat treating cast metal alloy parts using infrared energy as a heat source.